The present invention relates to a moisture-permeable disposable diaper. More particularly, the present invention relates to a disposable diaper comprising a substantially urine-impermeable backing sheet where the vapor permeation is adjusted to a specific level.
Absorbent pads are well known in the art and they are used for absorbing and retaining liquids discharged from bodies for example, as a diaper. It is known that the outside of such absorbent pads is covered with a plastic sheet to prevent the absorbed urine from passing through the absorbent pads and contaminating a garment or bedding. A known urine-impervious plastic sheet prevents the urine from passing through the absorbent pads and is effective for retaining the urine in the absorbent pads. However, the urine-impervious plastic sheet gives an unpleasant warm feeling to a wearer after absorption of the discharged urine or causes eruption or itching. Moreover, self-drying of the absorbent pads is inhibited by the presence of the plastic sheet.
It is therefore a primary object of the present invention to eliminate these defects of conventional disposable diapers and provide a disposable diaper having a substantially urine-impermeable backing sheet where the vapor permeation is adjusted to an appropriate level.
Another object of the present invention is to provide a disposable diaper having a substantially urine-impermeable backing sheet having a vapor permeation adjusted to an appropriate level, which can be manufactured very easily with economical advantages.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.
Pursuant to the present invention, there is provided a disposable diaper comprising a urine-permeable surface sheet, a urine-impervious backing sheet and an absorbent pad interposed between said two sheets, said disposable diaper being characterized in that the urine-impervious backing sheet comprises at least two layers, the first layer being a substantially urine-impermeable, vapor-permeable sheet, and the second layer being a urine-impermeable sheet which is perforated with an aperture ratio lower than 30%, said first and second layer being integrally formed.
A vapor-permeable, urine-impermeable sheet is ordinarily obtained by forming a great number of holes having a diameter smaller than 50.mu. on a sheet having a contact angle of at least 90.degree. to the urine. It is known that such a vapor-permeable backing sheet is used as a diaper cover, and woven fabrics composed of fibers having a fine pore, non-woven fabrics and perforated sheets, each having a high aperture ratio, have actually been used as such backing sheet. When such a sheet is used for a disposable diaper, since the area of pores occupies at least 50% of the total area of the sheet before perforation (that is, the aperture ratio is at least 50%) and the vapor permeability is too large, the outside of the backing sheet develops a wet feeling during wearing. This is not due to permeation of the discharge urine, but rather an unpleasant feeling is given to the wearer by the backing sheet. Accordingly, the aperture ratio must be reduced to control the moisture permeation. Of course, it is possible to obtain a backing sheet having a reduced moisture permeation from one film which is rendered urine-permeable by forming fine pores at an aperture ratio lower than 30%. However, such a film has not been economically available in the art.
According to the present invention, it was found that the vapor permeation of a diaper cover of two wool woven fabrics folded together corresponds to the vapor permeation of a polyethylene film on which pores having a diameter of 10 mm are formed at an aperture ratio of 30% or a polyethylene film on which pores having a diameter of 1 mm are formed at an aperture ration of 7 to 10%.
According to the present invention, the backing sheet comprises at least two layers which are integrally formed by thermal fusion bonding or the like. The first layer is substantially urine-impermeable, in which a great number of pores having a diameter smaller than 50.mu. are formed at a high aperture ratio, and the second layer is perforated at an aperture ratio lower than 30%, whereby the vapor permeation of the first layer is controlled. Each of the first and second layers can be formed so as to have a multi-layer structure and control the vapor permeation. In order to attain the objects of the present invention, it is preferred that the vapor permeation-adjusting layer (the second layer) be located on the outer side of the diaper. However, in this case, the appearance of the diaper is not good, and therefore, this layer may be located on the inner side. The vapor permeation cannot be controlled accurately unless the first and second layers closely contact each other, and therefore, the two layers should be integrated with each other by any conventional manner, for example by thermal fusion bonding or the like. Thus, for example, the first and second layers can be piled together and the assembly can be pressed by a hot roll.